The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions, typically disposed on catalyst supports or substrates, are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing CO and HC emissions is an oxidation catalyst device (“OC”). The OC device includes a flow-through substrate and a catalyst compound applied to the substrate. The catalyst compound of the OC induces an oxidation reaction of the exhaust gases once the OC device has attained a threshold or light-off temperature. One type of exhaust treatment technology for reducing NOx emissions is a selective catalytic reduction (“SCR”) device that may be positioned downstream of the OC device. An exhaust treatment technology in use for high levels of particulate matter reduction may include a particulate filter (“PF”) device that traps particulate matter, which may be positioned downstream of the OC device and the SCR device. Regeneration is the process of removing the accumulated particulate matter from the PF device.
In a typical arrangement of an exhaust gas treatment system, the OC device is located upstream of the SCR device and the PF device. Thus, the amount of heat that is lost between the OC device and the PF device may be significant, especially in exhaust gas treatment systems having relatively long exhaust pipes or if the exhaust gas flow is relatively low. The amount of heat that is lost between the OC device and the SCR device may be significant as well. In one approach to increase the amount of heat to the SCR and the PF device, the temperature of the OC device is continually increased. However, this approach may cause thermal shocks to the exhaust gas treatment system when quickly increasing the exhaust gas temperature. Moreover, this approach may also heat the OC device above a threshold temperature the OC device is intended to be able to withstand.
In another approach, the exhaust gas flow is increased to drive the heat from the OC device to the SCR and the PF device. This results in an increased PF temperature. However, as the exhaust gas flow in the exhaust gas treatment system increases, the temperature of the OC device will in turn decrease. Specifically, the temperature of the OC device will eventually drop to below the light-off temperature. This results in the OC device being unable to reduce CO and HC in the exhaust gas. Accordingly, there is a need for an exhaust gas treatment system having increased heat supplied to the SCR device and the PF device, while still maintaining the OC device at the respective light-off temperature.